The GenBank® sequence database, which is an annotated collection of all publicly-available nucleotide and amino acid sequences, contains sequences from approximately 30,000 species of bacteria. While this number may appear impressive, it is instructive to note that a recent estimate suggests that the sea may support as many as 2 million different species of bacteria, and a ton of soil more than double that number (Curtis et al., Proc. Natl. Acad. Sci. USA 99:10494-10499, 2002). Furthermore, only about 13,000 of the bacteria represented in GenBank® have been formally described, and almost all of these lie within 4 of the 40 bacterial divisions (DeLong, Curr. Opin. Microbiol. 4:290-295, 2001). The paucity of knowledge regarding other microbial species is similar or greater. This is at least in part due to the fact that the vast majority of microorganisms from the environment resist cultivation in the laboratory. These so called “uncultivables” represent 99-99.99% of all microbial species in nature (see, e.g., Young, ASM News 63:417-421, 1997).
Microbial diversity is typically examined by amplifying 16S rRNA genes from DNA samples isolated from a specific habitat. The sequences are then compared to each other and to the 16S rRNA sequences from known species. If no close match to an existing 16S rRNA gene sequence is found, then the test sequence is thought to represent a new microorganism and is termed an “uncultured microorganism.” 16S rRNA genes, which are critical for translation, are the genes of choice for these experiments because they are thought to be conserved across vast taxonomic distance, yet show some sequence variation between closely related species. Phylogenetic analyses of 16S rRNA sequences obtained from direct sampling of environments suggest that uncultured microorganisms can be found in nearly every taxon within Bacteria and Archaea, and several groups at the division level have been identified with no known cultivable representatives (see, e.g., Giovannoni et al., Nature 345: 60-63, 1990; and Dojka et al., Appl. Environ. Microbiol. 66:1617-1621, 2000).
The principal reason for this disparity is that few microorganisms from environmental samples grow on nutrient media in Petri dishes. The discrepancy between the microbial total count and plate count is several orders of magnitude. Attempts to improve the recovery of microorganisms from environmental samples by manipulating growth media have been of limited success. Accordingly, new methods for isolating and growing previously uncultivable microorganisms are desirable. These methods may be useful in identifying microorganisms that are a valuable resource of novel metabolic products useful for pharmaceutical and industrial processes. In addition, these methods may be useful in identifying microorganisms critical for decomposing and recycling nutrients at a global scale.